1. Field of the Invention
The present invention is directed to charge-coupled device (CCD) image sensors and more particularly to providing a technique and arrangement for achieving advantageous color filtering for CCD image sensors.
2. Prior Art
Image sensors using color-filter arrays (CFAs) with solid-state CCD arrays to capture color images electronically are generally well known in the art as indicated, for example, in the article entitled, xe2x80x9cFrom Photons to Bitsxe2x80x9d by R. P. KHOSLA in xe2x80x9cPHYSICS TODAYxe2x80x9d, December 1992, pps. 42-49. A very powerful color system has been used in these image sensors for some time, such as in color vidicons, e.g., SONY Trinicons, which system involves the use of diagonal criss-crossing stripes of Yellow and Cyan filters that form 2xc3x972-pixel blocks consisting of a Cyan, a Yellow, a Green, and a White (full response) pixel, with the Green being the result of a Cyan and Yellow overlap. A pattern for such a color filter array (CFA) is shown in FIG. 1 which illustrates an Hitachi complementary transmitted color CFA as disclosed by H. NABEYAMA ET AL., in xe2x80x9cIEEE Transactions on Consumer Electronicsxe2x80x9d, vol. CE-27, pp. 40-45, 1981. Complementary color CFAs of this type are typically used in low-light camcorder applications and have various forms. For instance, this layout design, which is used for interlaced television scanning, could be made using just a Cyan and a Yellow filter application, but the layout would not take the form of Cyan stripes or Yellow stripes. Further, FIG. 2 shows the Toshiba complementary frequency interleaving CFA with Green, Yellow, and Cyan pixels as disclosed by K. A. PARULSKI, in xe2x80x9cColor Filters and Processing Alternatives for One-Chip Camerasxe2x80x9d, IEEE Transactions on Electron Devices, vol. ED-32, pp. 1381-1389, August 1985, and FIG. 3 illustrates another color mosaic design, a xe2x80x9cGCMYxe2x80x9d complementary CFA, using Green, Cyan, Magenta, and Yellow, as disclosed by F. ASCHWANDEN ET AL., in xe2x80x9cSingle-Chip Color Camera Using a Frame-Transfer CCDxe2x80x9d, IEEE Transactions on Electron Devices, vol. ED-32, pp. 1396-1401. In the latter article by F. ASCHWANDEN ET AL., a xe2x80x9csymmetric shift complementary CFAxe2x80x9d is also disclosed of the form illustrated in FIG. 4.
Other examples of this general type of color filtering are disclosed in U.S. Pat. Nos. 4,450,475; 4,516,154; 4,580,160; 4,646,139; 4,721,999; 4,951,130; 5,028,547; and RE 32,492. There are also prior art filter arrangements wherein the colored stripes all are parallel to each other.
As to the solid-state CCD arrays, CCD imagers formed from solid-state arrays, as explained in the above-cited xe2x80x9cPHYSICS TODAYxe2x80x9d article, may be of the interline transfer (IT) type, composed of vertically stacked linear scanners, or of the frame transfer (FT) type, with each CCD pixel being a photosensing element in an integration-frame array that transfers captured charge to a storage-frame array. Another form is the full-frame (FF) type, wherein the pixels are generally square and their charge is read out individually. Existing CCD imagers are predominantly of the interline (IT) type, although frame transfer (FT) type CCD imagers have many high-performance advantages, such as higher resolution per pixel with less aliasing effect, but at a comparatively higher cost. The full-frame (FF) type is comparatively simpler than the IT and FT types as it does not use interlacing in its operation.
From another standpoint, there are two common types of area-array image sensors, i.e., 2-field-interlaced arrays and progressive-scan arrays. The first type to become commonly used was the interlaced arrays for television cameras, the predominant architecture of which was of the interline transfer (IT) type. The CFA designs for this type of imager were optimized for camcorder recorder bandwidths and generally are scanned so that in any one field all the rows are read out in pairs. The color mosaic design CFAs of the prior art have generally been used with field-interlaced television cameras or imagers where both fields must have essentially the same balance of color information. The simplest type of progressive-scan sensor is the full-frame (FF) type. For this type of sensor an optimized CFA will tend to be different. As with the interlaced arrays, however, the same CCD architectures can be used with the progressive-scan array, i.e., IT, FT, and FF. In both types of area-array image sensors it is desired to achieve high luminance and chrominance resolution, but in the progressive-scan type the design problem is not complicated by the feature of field-interlacing and matching of performance.
In one example of a large, high resolution image sensor, the yield in the fabrication of representative full-frame (FF) type CCD imagers without color filters typically runs about 2.4 good dice per wafer. But, with color filter-related steps this drops to about 1.2, i.e, the drop in yield may approach 50%. A color-filter imager fabricating process may typically use 6 masks, and have 5 patterned layers and 4 colors, with the last of these colors yielding at a much lower rate than the others. The present processes generally are too complex and expensive even assuming that there are 5 equally-yielding steps and no others.
Problem to be Solved:
It will therefore be seen that a CCD color imager would be desirable that could provide the highest resolution per pixel, have the least aliasing effect, and not be too expensive to manufacture.
Objects:
It accordingly is an object of the present invention to provide a method and means for making high performance color imagers in a relatively low cost way.
It is another object of the present invention to provide a method and means for fabricating high performance color imagers with an improved yield.
It is a further object of the present invention to provide a full-frame (FF) type CCD imager with improved color filtering that is comparatively less expensive and less complex to fabricate.
The present invention is directed to CCD image sensors with color filters for color imaging and involves depositing stripes of Cyan and stripes of Yellow filter material on the imaging surface of a CCD array in an image sensor, preferably a full-frame (FF) CCD imager or a progressive-scan frame-transfer CCD imager, and arranging these stripes perpendicular to each other and parallel to the array axes to form two layers at the areas where they cross. This arrangement can be used with appropriate spacing to form unit color cells comprising 2-pixel by 2-pixel blocks, with each block containing a Cyan, a Yellow, a Green (where the stripes overlap), and a White (where there is no stripe) pixel. Alternatively, the longitudinal or vertical stripes can be modified into segments of two pixels or more in length, and these segments may be displaced transversely or horizontally, i.e., staggered, so as to form an elongated checkerboard pattern. This variation in the arrangement has the advantage that, in a device that has single-column blemishes, the computed and interpolated pixel signal levels along such a blemish are on average more accurate and therefore these blemishes will not be as conspicuous.
Another feature of the invention involves a two layer filter stripe deposition technique wherein the upper stripe, i.e., later deposited stripe, can be fabricated so that its spectral properties in the areas or regions where it is deposited over the first deposited stripe are distinctly different from those regions where it is not deposited over the first stripe. This feature can be used to advantage to produce a four-color image sensor. For example, with the Yellow stripe on top, it is possible to distinguish between Blue and Violet so as to better handle the gamut of Bluish-Purples. Also, because there are only two color layers and because the dimensions of stripes are easier to control than the dimensions of pixel-sized squares that tend to have rounded corners and thus involve one more process control issue, the present two-stripe approach and arrangement is advantageous in the fabrication of an imager chip and in the yield obtained.
In operating the resulting full-frame CCD imager chip, the sensed image charge is read out after only one exposure. During or after readout, signal processing is done to generate the desired color image data file. The data file may be a Red-Green-Blue (RGB) file, a LUMINANCE-CHROMINANCES a and b file, or some other type of color image file. The Cyan-Yellow stripe color design has several unique color-output features. The unit cell, as noted, may be a block of 2 pixels by 2 pixels made up of a Green, a Cyan, a Yellow, and a White pixel. From one of these blocks, the Red level is {White (W) minus Cyan (Cy)}, and the Blue level is {White (W) minus Yellow (Ye)}. The LUMINANCE level is the sum of all four pixel levels. In between these 2-pixelxc3x972-pixel blocks, by virtually shifting one pixel both vertically and horizontally, there are complementary 2xc3x972 pixel blocks that can also be used to generate unique color image data.
A further arrangement using Cyan and Yellow filter material in the form of circular discs or octagons to form the 2-pixel by 2-pixel blocks containing the Cyan, Yellow, Green, and White pixels is also disclosed.